Recent uses of single crystal (SC) nickel-based superalloys engineered for aircraft engines or large power generation turbine components have shown that such alloys are prone to defects, such as freckles and stray grains. In the past, these defects have not been a large concern because of the relatively small dimensions of the directionally solidified components in aircraft engines or the relatively low operating stress and temperatures in power generation turbines. However, more recently it has been desired to increase the operating regimes of power generation turbine components to higher temperatures using thinner walls in an effort to improve turbine performance. Such operating regimes have resulted in freckle and stray grain defects being life-limiting in such power generation turbine components.
Broadly, the present invention is embodied in the addition of carbide-forming elements to nickel-based superalloys so as to minimize grain defects, such as freckle and stray grain defects. More specifically, the present invention involves the addition of carbide-forming elements that form from the liquid in the mushy zone of the solidification front of single crystal (SC) and directionally solidified (DS) nickel-based superalloys so as to reduce the formation of freckle and stray grain defects in such alloys.
In a particularly preferred embodiment of the present invention, a nickel-based superalloy is provided which includes, by weight, between about 4.00% to less than 9.25% tantalum, between about 4.75% to about 6.50% tungsten, at least about 2.75% rhenium, between about 5.00% to 7.00% aluminum, at least about 0.10% hafnium and carbon in an amount sufficient to form carbides with other constituents to reduce significantly freckle formation in the mushy zone of the superalloy during casting.
The alloys of the present invention are especially useful to form cast turbine blades, particularly blades for power generation turbines. The castings can be used "as is", or may be subjected to further processing, such as high temperature solution and aging treatments, coating treatments and the like.
These and other aspects and advantages of the present invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.